Method for producing cold workable hypoeutectoid steel

ABSTRACT

Hypoeutectoid steels are worked within a temperature range of between the A1 temperature to 150* F. below the A1 temperature. The cross-sectional area of the steels is reduced by not less than 60 percent during working. After working, the steels can be heated to about the A1 temperature to obtain the optimum hardness and ductility for cold-working. The as-worked structure and the heat treated structure are also described.

United States Patent Kranenberg Oct. 2, 1973 METHOD FOR PRODUCING 3,459,599 8/1969 Grange 143 12 COLD WORKABLE HYPOEUTECTOID 2,814,578 II/I957 White I48/I2 STEEL FOREIGN PATENTS OR APPLICATIONS [75] Inventor: Helmut Kranenberg, Bethlehem, Pa. 1,161,234 8/1969 Great Britain [73] Assignee: gellrllinllem Sltaeel Corporation, Primary Examiner w w. stauard ct e Attorney-Joseph J. OKeefe [22] Filed: Apr. 10, I972 211 App]. No.1 242,473 [57] ABSTRACT Hypoeutectoid steels are worked within a temperature range of between the A temperature to 150 F. below [52] :LS. CI. the A] temperature The cross sectional area of the [51] 48/12 steels is reduced by not less than 60 percent during [58] Fred 0 Search l working. After working the steens can be heated to 6 about the A temperature to obtain the optimum hard- [56] I References and ness and ductility for cold-working.

1 UNITED STATES PATENTS The ,as-worked structure and the heat treated structure 3,076,361 2 1963 Epstein 6! al. 148 12 are also described 3,285,789 11/1966 Grange et al. I 3,423,252 H1969 Grange l48/l2 9 Claims, 6 Drawing Figures PRIOR ART HEAT TREATING CYCLE SPHEROIDIZED OF THE INVENTION LING YCLE 5*- I400 5 I 300 END OF PRIOR ART E I SPHEROIDIZED 1 I200 ANNEALING CYCLE END OF Lu HEAT TREATING CYCLE I I 00 OF THE INVENTION LU p. [000 I I I I O 2 4 6 8 IO I2 I4 l6 I8 ANNEALING TIME hrs.

PAIENTED 2 I973 sum 10F 2 Fig.3

F'MENTEU 2'975 3.762.964

SHEET 2 F 2 PRIOR ART VICKERS HARDNESS NO.

TEMPERATURE SPHEROIDIZED HEAT TREATING CYCLE ANNEALING CYCLE OF THE INVENTION I300 END OF PRIOR ART SPHEROIDIZED I200 END OF ANNEALING CYCLE HEAT TREATING CYCLE I I 0 OF THE INVENTION 000 I 1 1 I 1 1 I 1 o 2 4 s a I2 14 I6 18 ANNEALING TIME m.

230 220 a I o HARDNESS OF WORKED STEEL 0 WHEN HEAT TREATED BY 20o CYCLE OF THE INVENTION HARDNESS OF CONVENTIONALLY I 0 WORKED STEEL WHEN SPHEROIDIZE ANNEALED BY '80 CONVENTIONAL CYCLE -n-un-. I30

o 2 4 s s 10 I2 I4 I6 18 ANNEALING TIME h rS.

METHOD FOR PRODUCING COLD-WORKABLE HYPOEUTECTOID STEEL BACKGROUND OF THE INVENTION This invention is in general directed to a method for working hypoeutectoid steels within a temperature range to effect a desired reduction in cross-sectional area. The hypoeutectoid steels can be heated to a temperature for a time to obtain optimum hardness and ductility suitable for cold-working. More specifically, the invention is directed to a method for working and heat treating hypoeutectoid steels, for example, steels containing about 0.30 to about 0.80 percent carbon, wherein said steels are worked to reduce the crosssectional area by not less than 60 percent within a temperature range of about the A, temperature to about 150 F. below the A, temperature. The steels are subsequently heated to about the A, temperature to obtain a hardness and ductility suitable for cold-working, such as cold-heating and the like.

Steels used for cold working into various products, such as bolts, screws and the like, generally are of the hypoeutectoid type containing up to about 0.80 percent carbon.

Prior art practice in manufacturing cold-workable hypoeutectoid steels is to refine the steel in a metallurgical furnace, such as an electric furnace and the like, tap and teem the steels into ingot molds to form ingots. The ingots are hot-worked at an austenitizing temperature into the end product, such as bars, billets, rods, wire and the like. The end product is slow cooled to ambient temperature. The steels are spheroidize annealed to obtain a uniform structure which is substantially completely spheroidized; is substantially completely free of a carbide network and pearlite; and is relatively soft and ductile. Because of the duplex structure and high hardness of the steels after slow-cooling from the hot-working temperatures, the spheroidize anneal cycles which are used are lengthy. The steels must be soaked at the proper temperature for from about hours to days to completely and effectively produce the desired microstructure and hardness and ductility re quired for good cold formability. In order to decrease the time of annealing, the steels are alternately heated and cooled to a few degrees of temperature above and below the A, temperature several times. Although the microstructure and hardness produced by the practices are acceptable by present day standards, the cyclic heating and cooling practice does not effectively reduce the length of the annealing cycle. Then, too, the microstructure of hypoeutectoid steels treated by the cyclic heating and cooling method can contain evidence of lamellar carbides. The hardness of the steels is reduced to a hardness suitable for cold-working after lengthy time at the spheroidize annealing temperature but maximum reduction in hardness may not be achieved.

Recently several'improvements in the manufacture of cold-workable steels have been suggested. One such improvement is U.S. Pat. No. 3,285,789 issued Nov. l5, l966 to Raymond A. Grange et al. titled Method of Softening Steel. The improvement is directed to heating hypoeutectoid steels to temperatures wherein the steels are completely austenitic, working the steels while at these temperatures, cooling the steels to ambient temperature and spheroidize annealing the worked steels within a temperature range between the A, temperature to F. below the A, temperature. The structure obtained after spheroidize annealing is satisfactory by present standards. Complete spheroidization and the eliminationof lamellar carbides is not achieved in the as-worked condition.

The as-worked steels contain ferrite and pearlite, therefore the steels must be spheroidize annealed to produce a spheroidized structure.

Another improved method is directed to hypereutectoid steels and highly alloyed steels as described in U. S. Pat. No. 3,459,599 issued Aug. 5, 1969 to Raymond E. Grange titled Method of Thermomechanically Annealing Steel." Hypereutectoid steels are heated to and drastically worked at a temperature not more than 150 F. above the A, temperature and ore finished below the A, temperature but not more than 50 F. below the A, temperature. The method, while applicable to hypereutectoid steels, is not applicable to hypoeutectoid steels. The problems of complete spheroidization with elimination of lamellar carbides connected with hypoeutectoid steels is not solved.

Although prior art practices have been developed to roll steels ferritically as disclosed in U. S. Pat. No. 3,076,361 issued Feb. 5, 1963 to S. Epstein et a1. titled Rolling Steel in Ferritic State," the high alloy and tool steels to which the process is directed, must be heat treated, for example, spheroidize annealed, prior to heating and working. The special heat treatment prior to heating for working and controlled heating for working increase the cost of the production of the steel.

SUMMARY OF THE INVENTION It is an object of this invention to provide a method for producing hypoeutectoid steels having good formability wherein said steels are worked within a temperature range below the A, temperature and the worked steels are heat treated at about the A, temperature for a time to decrease the hardness and increase the ductility of the steels.

It is an object of this invention to provide a method for producing hypoeutectoid steels suitable for coldworking wherein said steels are worked within a temperature range between about the A, temperature to about 150 F. below the A, temperature and the worked steels are heat treated at about the A, temperature for a time to obtain a desired hardness and ductility.

It is an object of this invention. to provide a method for producing as-worked hypoeutectoid steels wherein said steels are worked within a temperature range between about the A, temperature to about 150 F. below the A, temperature to obtain a microstructure of fine well-dispersed spheroidal carbides in a fine ferritic matrix substantially devoid of lamellar carbides.

It is an object of this invention to provide a method for producing hypoeutectoid steels suitable for coldworking, said steels containing about 0.30 to about 0.80 percent carbon, wherein said steels are worked within a temperature range between about the A, temperature to 150 F. below the A, temperature for a time to reduce the cross-sectionzal area therof by not less than percent and the worked steels are heat treated at about the A, temperature for up to about six hours to produce a microstructure consisting of welldispersed spheroidal carbides in a ferritic matrix, said steels being characterized by low hardness and good ductility.

Broadly, the invention includes working hypoeutectoid steels within a temperature range of about the A, temperature to about 150 F. below the A, temperature to reduce the cross-sectional area by not less than 60 percent and to heat treat the steels at about the A, temperature for a time to obtain a low hardness and ductility.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a reproduction of a photomicrograph taken at 2200 diameters of a hypoeutectoid steel as-worked by the method of the invention.

FIG. 2 is a reproduction of a photomicrograph taken at 2200 diameters of a hypoeutectoid steel as-worked and heat treated by the method of the invention.

FIG. 3 is a reproduction of a photomicrograph taken at 500 diameters of a hypoeutectoid steel as-worked and heat treated by the method of the invention.

FIG. 4 is a reproduction of a photomicrograph taken at 500 diameters of a hypoeutectoid steel as-worked and spheroidize annealed by a conventional method.

FIG. 5 is a graph comparing a conventional method of spheroidize annealing and the method of heat treating of the invention.

FIG. 6.is a graph showing a comparison of the decrease in hardness of steels worked and spheroidize annealed by a conventional method and by the method of the invention.

PREFERRED EMBODIMENT OF THE INVENTION Hypoeutectoid steels suitable for cold-working, for example, cold-forming, cold-threading and the like, can be made by working the steels within a temperature range below about the A, temperature and heat treating the worked steels at about the A, temperature for a time to obtain optimum hardness and ductility. The as-worked hypoeutectoid steels have a microstructure of fine spheroidal carbides well-dispersed in a fine ferritic matrix substantially devoid of lamellar carbides. The heat treated steels have a microstructure of somewhat larger spheroidal carbides well dispersed in a ferritic matrix substantially devoid of lamellar carbides.

In the practice of the invention, hypoeutectoid steels are melted and refined in any type of metallurgical furnace, such as basic oxygen furnace, electric furnace, open-hearth and the like. The refined steels are tapped into a ladle and teemed into ingot molds in the conventional manner. The ingots thus formed are heated to an austenitizing temperature and are rolled into billets and cooled to black. At this stage of processing the steels, it is possible to pursue either one of two steps: (1) the billets can be heated to an austenitizing temperature and worked at the austenitizing temperature to effect a reduction in the cross-sectional area, said reduction is of such a nature that the steels will require additional working at a temperature to effect at least another 60 percent reduction in cross-sectional area to obtain the final size desired, after which the billets are cooled rapidly, for example, in air through the A -A, temperature range to a temperature range between the A, temperature and 150 F. below the A, temperature, in which temperature range the additional reduction in cross-sectional area is achieved, or (2) the billets can be reheated to a temperature range between about the A, temperature and 150 F. below the A, temperature and the steels worked within this temperature range to the desired final size. Whichever of the two above steps is taken, the hypoeutectoid steels are worked within a temperature range of between about the A, temperature to 150 F. below the A, temperature to obtain not less than 60 percent reduction in crosssectional area to achieve the results of the invention. After working, the hypoeutectoid steels can be heat treated for a time at about the A, temperature. It will be understood that wherever heat treatment is used in the specifications and claims in regards to the steels processed by the method of the invention such heat treatment includes heating the steels to about the A, temperature for a time to reduce the hardness of the steels and to increase the ductility of the steels with little or no effect on the spheriodization microstructure of the steels other than a slight increase in the size of the carbides and ferritic matrix. It will also be understood that to raise the temperature of the steels to the working temperature range between about the A, temperature and 150 F. below the A, temperature it is possibleto heat the steels above the A, temperature or even above the A, temperature so long as the steels are cooled to within the working temperature range described above before any reduction in cross-sectional area is started. Although working the steels within the temperature range of about A, temperature to about 150 F. below the A, temperature will achieve the results of the invention, good results can be achieved by working the steels within a temperature range of about 5 F. to about F. below the A, temperature and better results can be achieved by working the steels within a temperature range of about 75 F. to about F. below the A, temperature. It is, therefore, preferred to work the steels within a temerature range of about 75 F. to about 150 F. below the A, temperature.

The hypoeutectoid steels are worked within the temperature ranges mentioned above to obtain a reduction in the cross-sectional area of not less than 60 percent. Samples of the steels as-worked by the above described method were examined by electron microscopy at a magnification of 2200 diameters. The microstructure was found to consist of fine spheriodal carbides welldispersed in a fine-grain ferritic matrix. A reproduction of an electron photomicrograph of the structure at the latter magnification is shown in FIG. 1. It can be seen that the carbides are well spheriodized and are less than 1 micron in size when compared with a line 5 microns long drawn on the lower right-hand corner of the elec tron photomicrograph for comparison purposes. The ferritic grains are also small, not more than 1 .5 microns in size, although they appear to be very large when compared to the carbides. The microstructure is substantially free of lamellar carbides.

The hypoeutectoid steels were heat treated at about the A, temperature for from about three hours to about six hours. A reproduction of an electron photomicrograph of a sample of the steels after heat treatment taken at a magnification of 2200 diameters is shown in FIG. 2. The spheriodal carbides and ferrite grains have been coarsened by the heat treatment. The carbides are less than 5 microns in size when compared to a line 5 microns in length drawn in the lower right-hand corner of the photomicrograph for comparison purposes.

A microscopic examination at a magnification of 500 diameters of the steels after heat treatment is shown in FIG. 3. The microstructure can be seen to consist of finely divided spheriodal carbides well dispersed in a ferritic matrix. The microstructure is substantially free of carbide network, and lamellar carbides.

A microstructure at a magnification of 500 diameters typical of hypoeutectoid steels processed by a conventional method of producing hypoeutectoid steels, that is, hot rolling at austenitizing temperatures, for example, 1550 F., and spheriodize annealing by a conventional annealing cycle wherein the steels are cyclically heated and cooled a few degrees in temperature above and below the A temperature for about 17 hours, is shown for comparison purposes in FIG. 4. The carbides are tending to spheriodize but a large portion thereof retain lamellar-like formations and are not welldispersed. Ferrite grains are outlined by the carbides. The ferrite grains appear to be larger in the conventionally worked and spheriodize annealed steels than the ferrite grains of the steels worked and heat treated by the method of the invention shown in FIG. 3.

The steels of the invention were tested for hardness both in the as-rolled and heat treated conditions. The as-rolled steels with a microstructure shown previously in FIG. 1 were found to have a hardness of 200 DPI-I (Vickers) to about 230 DPI-I (Vickers) which is equiv alent to a hardness within a range of about 190 BHN to about 220 BI-IN. The hardness range is above the hardness desired in steels which are to be cold-worked. After heat treating for a time, about three hours to about six hours, by the method of the invention, the steels had been lowered in hardness by about 80 points in both DPI-I (Vickers) and BI-IN, which is well within the hardness range for cold-working the steels, for example, cold-heading.

A comparison of the short heat treating cycle of the invention and a typical conventional spheriodize annealing cycle is shown in FIG. 5. Note that the heat treating cycle of the invention is considerably shorter than the typical conventional spheriodize annealing cycle.

FIG. 6 is a comparison of the effect of the heat treating cycle of the invention on the as-worked hardness of hypoeutectoid steels and a typical conventional spheriodize annealing cycle in the as-worked hardness of the hypereutectoid steels. It will be noted that the asworked hardness of the hypoeutectoid steels of the invention was higher than the as-worked hardness of hypoeutectoid steels prepared by a conventional hot working process. However, the hardness of the hypoeutectoid steels worked by the method of the invention decreased much more rapidly when heat treated than the hardness of the hypoeutectoid steels worked by conventional hot working process. In fact, the hardness of the hypoeutectoid steels of the invention after heat treating at about the A, temperature for about six hours is comparable to the hardness of hypoeutectoid steels hot rolled by conventional hot rolling and spheriodize annealed by conventional annealing cycle for about 17 hours. As seen in the dotted line, the hardness of the hypoeutectoid steels of the invention is lowered slightly when heat treated for longer periods of time. At each interval of time the hardness of the hypoeutectoid steels of the invention is lower than the hypoeutectoid steels prepared by a conventional hot rolling ahd spheriodize annealing cycle.

The as-worked hardness of the hypoeutectoid steels of the invention may be sufficiently high to preclude cold forming, however the tensile stength and reduction in area of these steels are better than conventionally processed steels of the same grade. Therefore. in some applications, the hypoeutectoid steels of the invention can be used in the as-worked condition.

It will be understood that wherever percentages are mentioned in these specifications and claims, such percentages are on a weight basis unless otherwise noted.

In a specific example of the invention, a hypoeutectoid steel having a chemical analysis of: carbon 0.39%, manganese 0.75%, phosphorus 0.017%, sulfur 0.022%, silicon 0.18% was prepared in a basic oxygen furnace. The steel was melted, poured and teemed into 34 inch 4; ingot molds. The ingots were bloomed to 4 inches by 4 inches square billets and cooled to ambient temperature. The billets were reheated to austenitizing' temper ature and reduced in size to 246 inches X l-B inches billets, finished at 1900 F. and allowed to drop in temperature to about 1200 F. in air. The billets were reduced in cross-sectional area by 60.4 percent to l-% inches in diameter. The rounds were air cooled to ambient temperature. Microscropic examination at a magnification of 22000 diameters of samples cut from the bars showed a microstructure of fine, well-dispersed spheriodal carbides of about 0.1 to 0.3 microns in size in a fine-grained ferritic matrix of about 0.5 to 1.5 microns in size, devoid of lamellar carbides. The bars were heated in a furnace at a temperature of about 1300 F. for five hours. The bars were slow cooled to room temperature.

Microscopic examination at a magnification of 8200 diameters of the steel after heat treatment disclosed a microstructure of well-dispersed spheriods of carbides of about 0.5 to 2.5 microns in size in a ferritic matrix of about 3 to 10 microns in size. The hardness of the as-rolled bars was 229 DPH (Vickers). After heat treating, the hardness was 144 DPH (Vickers). Tensile tests of as-rolled bars showed the steels to have a tensile strength of 101,000 pounds per square inch and a reduction-in-area of 68 percent. The reduction-in-area compares favorably to steels processed by prior art methods which had a tensile strength of 89,000 pounds per square inch and a reductionin-area of 63 percent.

The bars were heat treated at 1300 F. for live hours. Tensile tests showed the steels to havea tensile strength of 73,000 pounds per square inch and a reduction-inarea of 79 percent. The ductility of the bars prepared by the method of the invention had improved ductility as compared to the conventionally treated bars.

I claim:

l. A method for producing hypoeutectoid steel characterized by having good cold formability comprising:

a. working said steel within a temperature range of about the A, temperature to about 150 F. below the A, temperature to reduce the cross-sectional area thereof, and

b. heating the steel for a time at about the A,

temperature.

2. The method of claim 1 in which the temperature range of step (a) is about 5 F. to about F. below the A, temperature.

3. The method of claim 1 in which the temperature range of step (a) is about 75 F. to about F. below the A, temperature.

4. The method of claim 1 in which the reduction in subparagraph (a) is not less than 60 percent.

5. The method of claim 2 in which the reduction in subparagraph (a) is not less than 60 percent.

which is between about F. to about l50 F. below the A, temperature to reduce the crosssectional area by not less than about 60 percent, b. cooling said steel to ambient temperature,

5 c. heating said steel at about the A temperature for a time to reduce the hardness and increase the duetility thereof.

9.- The method of claimv8 in which the time of heating [0 in step (c) is between about three hours and about six hours. 

2. The method of claim 1 in which the temperature range of step (a) is about 5* F. to about 75* F. below the A1 temperature.
 3. The method of claim 1 in which the temperature range of step (a) is about 75* F. to about 150* F. below the A1 temperature.
 4. The method of claim 1 in which the reduction in subparagraph (a) is not less than 60 percent.
 5. The method of claim 2 in which the reduction in subparagraph (a) is not less than 60 percent.
 6. The method of claim 3 in which the reduction in subparagraph (a) is not less than 60 percent.
 7. The method of claim 1 in which the hypoeutectoid steel is a steel which contains between about 0.30 to about 0.80 percent carbon.
 8. In a method for producing a hypoeutectoid steel containing about 0.30 to about 0.80 percent carbon, characterized by having low hardness and good ductility and a microstructure comprising well-dispersed, spheriodal carbides in a ferritic matrix, said method comprising: a. working said steel within a temperature range which is between about 5* F. to about 150* F. below the A1 temperature to reduce the cross-sectional area by not less than about 60 percent, b. cooling said steel to ambient temperature, c. heating said steel at about the A1 temperature for a time to reduce the hardness and increase the ductility thereof.
 9. The method of claim 8 in which the time of heating in step (c) is between about three hours and about six hours. 